1. Field of the Invention
This invention relates to a feed forward arrangement for linearising a distorting element. In particular, the distorting element may be an amplifier, with the combination being referred to as a feed forward amplifier.
2. Description of Related Art
In an ideal system, a linear amplifier provides uniform gain throughout its dynamic range in order that the output signal of the amplifier is a correct, amplified version of the input signal. In reality however all linear amplifiers exhibit non-ideal properties such as amplitude and phase distortion, which are undesirable and can seriously deteriorate the performance of a system. One effect of this non-linearity of the amplifier is the generation of output frequencies equal to the sums and differences of integer multiples of the input frequency components. This effect is known as intermodulation distortion (IMD) and is particularly undesirable in high-power radio frequency (RF) amplifiers designed for use in broadband systems. For example, a broadband amplifier used in the TDMA cellular system will generate various intermodulation products as a result of amplifying a multitude of TDMA channels occurring at fixed frequency intervals across a TDMA band, with coincident active frames.
A number of linearisation techniques have been developed to overcome the above distortion problems associated with a linear amplifier. A few of these techniques operate in real-time to account for time dependent changes in the non-linear characteristics of the amplifier. Such changes may result from, for example, temperature variations in the amplifier, ageing of amplifier components, power supply fluctuations, or, most particularly, changes in the operating point of the amplifier due to a change in the number or power of the input carriers. Of the broadband, RF-based linearisation techniques, the two most commonly used are feed forward linearisation and predistorter linearisation.
In general, predistortion linearisation mechanisms involve deliberate alteration of the relatively low level input signal to the amplifier in anticipation of the undesired distortion process occurring within the amplifier. Specifically, the mechanism predistorts the input signal in an inverse sense to the distortion produced by the amplifier such that in series the overall distortion is minimised. Accordingly, the transfer characteristic of the predistorter is approximated as closely as possible to the inverse or complementary function of the transfer characteristic of the amplifier. If the linear amplifier is compressive, i.e. the gain tails off at higher power levels, then the predistorter will compensate for this compression by correspondingly expanding the input signal.
Feed forward linearisation mechanisms rely on creating an error signal representative of the IMD products introduced by the linear amplifier, and feeding this signal forward to combine with the output spectrum of the amplifier, cancelling out the unwanted distortion. In order for the cancellation process to operate correctly, it is necessary for the mechanism to accurately adjust the amplitude and phase of the error signal prior to combining it with the output of the amplifier. To this end, it is known to use pilot signals in the feed forward mechanism as a way of controlling the amplitude and phase adjustments of the error signal. The pilot signals are artificially generated and are added with the amplifier signals to form an artificial xe2x80x9cdistortionxe2x80x9d component of known magnitude and frequency. By monitoring the presence of this artificial distortion in the corrected output spectrum of the amplifier, the errors in the cancellation process can be measured and accounted for by appropriate adjustment of the amplitude and phase of the error signal.
Different types of pilot signals are generated by the feed forward mechanism depending on the category or class of amplifier used/or and the form of input signal to be amplified. For example, in a spread spectrum system, the pilot signal may be chosen to have a frequency range which corresponds to the spread spectrum input signal. In contrast, in a TDMA system, the pilot signal may be chosen such that it fits in the gaps of the input signal frequency spectrum, which will depend on, for example, the channel width and channel spacing of the signal. The appropriate pilot signal is typically generated in a frequency synthesiser which requires control and adds to the feed forward mechanism complexity.
According to a first aspect of the present invention there is provided a feed forward arrangement for linearising a distorting element, the feed forward arrangement having error correction means in which a reference signal from the distorting element input is compared with the distorting element output signal to produce a first error correction signal, which is combined with the output signal to produce a corrected output signal, and in which a pilot signal is injected into the distorting element path so as to be processed by the error correction means together with the distorting element output signal, the presence of the pilot signal in the corrected output signal being monitored by comparison with the pilot signal so as to produce a second error correction signal, characterised in that the pilot signal is derived from the distorting element input signal or any equivalent signal.
A feed forward arrangement in accordance with the invention has an advantage that acquires the relevant information for the pilot signal directly from the input signal. Therefore, the arrangement does not need to have any prior knowledge about the form, channel spacing or bandwidth of the input signals in order to produce an appropriate pilot signal. A generic hardware implementation of the feed forward arrangement can therefore be produced, for example, in an application specific integrated circuit (ASIC), which can be employed in a variety of frequency bands or systems.
Another advantage of a feed forward arrangement in accordance with the invention is that it does not require any upconversion synthesisers which are normally necessary to generate and detect a pilot signal in a correct frequency band for the distorting element. The pilot signal can automatically concentrate on the area of the spectrum in which the input signals appear, providing maximum benefit where it is needed. If, for example, the input signals were concentrated in the lower half of a designated band of operation, then the feed forward arrangement would automatically be optimised for use in that part of the band. In contrast, prior art solutions are in general optimised in a predetermined portion of the band such as, for example, the whole of the band for a spread spectrum based pilot signal.
A further advantage of a feed forward arrangement in accordance with the invention is that the bandwidth of the pilot signal can be automatically related to the bandwidth of the input signal(s). In this way, the pilot energy will be concentrated in the area of spectrum where it is required and does not spread outside that region. Consequently, it is not necessary to remove any pilot signal appearing outside of the band (by filtering or otherwise) which can result in cost reduced system.
In one embodiment in accordance with the first aspect of the invention, the pilot signal is derived by modulation of the input signal with a modulating signal. Alternatively, the pilot signal can be derived as a third order or higher component of the input signal. Suitably, the input signal is attenuated in the pilot signal.
In a preferred embodiment, a frequency offset is applied to the pilot signal prior to the pilot signal being injected into the distorting element path. This frequency offset can be applied by mixing the pilot signal with a relatively low frequency tone signal from a local oscillator.
Preferably, the distorting element comprises an amplifier such as a broadband radio frequency power amplifier.
The error correction means can include means for adjusting the first error correction signal in dependence on information contained in the second error correction signal, and the adjustment means can enable adjustment of the first error correction signal in phase and amplitude. Accordingly, the adjustment means may comprise a variable phase shifter and a variable attenuator, or it may comprise an in-phase adjustment means and a quadrature phase adjustment means.
The feed forward arrangement may provide for dual feed forward correction whereby the feed forward arrangement has a second error correction means, independent of the first error correction means, which produces a third error correction signal for combining with the distorting element output signal to further correct the output signal, and in which a second pilot signal is injected into the distorting element path, the presence of the second pilot signal in the further corrected output signal being monitored by comparison with the second pilot signal so as to produce a fourth error correction signal, wherein the second pilot signal is also derived from the distorting element input signal.
According to a second aspect of the present invention there is provided a method for linearising a distorting element, including a comparing step in which a reference signal from the distorting element input is compared with the distorting element output signal to produce a first error correction signal, a combining step in which the first error correction signal is combined with the output signal to produce a corrected output signal, and an injecting step in which a pilot signal is injected into the distorting element so as to be processed together with the distorting element output signal by the comparing step, the presence of the pilot signal in the corrected output signal being monitored by comparison with the pilot signal so as to produce a second error correction signal, characterised in that the pilot signal is derived from the distorting element input signal or any equivalent signal.
According to a third aspect of the present invention there is provided an apparatus for correcting distortion generated in a distorting element, comprising comparing means for coupling to an input and an output of the distorting element for comparing a signal from the element input with a signal from the element output to produce an error signal, correcting means, coupled to the comparing means, for adjusting the error signal to produce a correction signal which is combined with the output signal of the element to produce a corrected output signal, means for deriving a distortion signal from the distorting element input signal, which distortion signal is a third order or higher component of the input signal, and control means for comparing the corrected output signal with the pilot signal so as to produce a control signal which controls the adjustment of the error signal in the correcting means.
According to a fourth aspect of the present invention there is provided a method for correcting distortion generated at an output of a distorting element, comprising the steps of comparing a signal from the distorting element input with a signal from the distorting element output to produce an error signal, adjusting the error signal to produce a correction signal which is combined with the output signal of the distorting element to produce a corrected output signal, deriving a pilot signal from the distorting element input signal, which pilot signal is a third order or higher component of the input signal, and comparing the corrected output signal with the pilot signal so as to produce a control signal which controls the adjustment of the error signal in the correcting means.
Further features and advantages of the invention will be apparent from the description below.